1. Field of the Invention
This invention relates to device manufacture and, in particular, to device manufacture involving an etching procedure.
2. Art Background
During the fabrication of devices such as silicon integrated circuits, especially at design rules finer than 1 .mu.m, it is often desirable to smooth, i.e. locally planarize, a relatively nonplanar surface. For example, the formation of active devices in the n and p tubs of a CMOS device configuration yields a relatively nonplanar surface. An oxide such as a chemical vapor deposition (CVD) oxide formed from a tetraethoxysilane (TEOS) precursor is deposited on this surface to electrically isolate it from an overlying metal interconnect pattern. The resulting deposited layer also has a nonplanar surface corresponding to that of the underlying layers. (Underlying refers to a material closer to the original substrate). The required subsequent deposition and patterning of an aluminum layer on this relatively irregular surface is difficult at design rules in the range 1 .mu.m and essentially unacceptable at design rules finer than 0.8 .mu.m. Therefore, it is desirable or essential to smooth the surface to produce an overlying aluminum pattern of appropriate configuration.
One typical procedure used for smoothing involes plasma etchback. In this process, a material with similar plasma etching characteristics to the layer being smoothed is deposited in a sufficiently thick layer to yield a smoothed upper surface. (A smoothed upper surface in the context of this disclosure has, over at least 90% of the area being smoothed, a surface that does not deviate more than 10 degrees, lesser included angle, from an imaginary plane parallel to the surface of the substrate before device processing.) The composite structure is then plasma etched; etching is continued until the entire smoothing layer is removed. Since the smoothing material and the underlying material are etched at essentially the same rate, the surface of the underlying material after essentially total removal of the smoothing layer takes on a surface configuration similar to the exposed surface of the smoothing material before etching, that is, the smooth surface of the smoothing layer is transferred by plasma etching to the underlying region.
Plasma etching is employed since materials typically employed for etchback smoothing are polymers such as novolac photoresists that have a similar etching rate in a plasma, e.g. a plasma containing fluorine and oxygen entities, to typical underlying material such as phosphorus and/or boron-doped CVD oxide. Despite the acceptable results by plasma etchback smoothing, this procedure requires the relatively large capital investment and maintenance costs associated with plasma etching equipment. Further, polymer smoothing materials often contain low levels of ionic impurites that tend to remain after plasma removal and require a subsequent cleaning procedure with its concomitant cost. Additionally, localized effects associated with plasma etching of organic materials generally yield ethcing-rate non-uniformities of typically 5% or more across a wafer and from wafer to wafer resulting in either under or over etching of the underlying layer. Thus, the etchback procedure as presently practiced has certain disadvantages.